The dead hostage apparently succumbed to a lingering illness, military southern command spokeswoman Capt. Jo-Ann Petinglay said.
The military launched the rescue while President Rodrigo Duterte was visiting Vietnam for the Asia-Pacific Economic Cooperation (APEC) Summit.
Petinglay identified the rescued hostages as?Buy Xuan Vien, Bui Trung Duc, Nguten Quang Huy, and Nguyin Huu Trong, and the dead as Hu Trung.
All were crew members of the cargo ship MV Giang Hai 5, which was boarded by Abu Sayyaf gunmen as it passed through waters off Sulu Island in the south last February.
The Vietnamese were among 17 crew members, including two who were also killed when they fought off the gunmen. Ten others were able to escape, officials said.
"Due to a lingering illness, Huu Trong succumbed to death during his captivity," Petinglay said, without elaborating.
The rescue operation was carried out in the early evening of Friday, near the island of Kang Tipayan in the remote island province of Tawi-Tawi, in the country's far south, the military said.
In September, troops rescued two Indonesian hostages also held by Abu Sayyaf militants in the jungles of southern Jolo Island. Seven other foreigners are believed still in Abu Sayyaf hands in the south, according to the military.
The Abu Sayyaf, or Bearers of the Sword, is the most violent of several Muslim rebel factions in the southern Philippines. It is responsible for the worst attacks in the country, including a ferry bombing in Manila Bay that killed more than 100 people in 2004.
The gunmen beheaded an elderly German captive earlier this year after his government failed to fork over ransom, while two Canadians were also killed in the same manner last year.
Military regional chief Lt. Gen. Carlito Galvez said ground units would push ahead with its anti-terror operations against the bandits "to thwart their kidnapping ploys and rescue their victims."
Another Abu Sayyaf faction led by Isnilon Hapilon, the acknowledged leader of the Islamic State in the region, engaged government forces in a five-month battle that destroyed much of Marawi city, also in the south. The vicious firefights killed 930 militants, 165 soldiers and 47 civilians, officials said.
In August, naval counter-terrorism troops also rescued Do Trung Hieu, a Vietnamese sailor held hostage by Abu Sayyaf militants for nine months on Basilan island, about 1,500 km (875 miles) south of Manila.
On Wednesday, six soldiers were killed and four wounded when the military clashed with the Abu Sayyaf in Basilan, in what is considered the single deadliest encounter for the army since security forces regained control of Marawi in October.
Duterte skipped a gala dinner in Vietnam after he was told of the military's rescue operation.
"He is supervising and coordinating (the) safe recovery of Vietnamese captives of the Abu Sayyaf group," Roque said in a short statement.
Reported by BenarNews, an RFA-affiliated online news service.
Copyright 1998-2014, RFA. Published with the permission of Radio Free Asia, 2025 M St. NW, Suite 300, Washington DC 20036